Various methods exist for storing video and images on computer storage such as hard drives, flash drives, CDs, DVDs, BLU-RAY and the like. It is usually desirable to compress the data making up a video or image before storing in order to avoid using too much storage space to store a particular video segment or one or more photographic or their graphic images, particularly if they are high definition or high resolution, which tend to involve more data.
U.S. Pat. No. 8,462,194 discloses a method for processing High Dynamic Range (HDR) images, which provides for generating a Low Dynamic Range (LDR) image by clamping the HDR image. The method, disclosed as “Fifth method” in U.S. Pat. No. 8,462,194, also provides for generating a fractional colour representation (called FC-frame) of the LDR image by dividing each RGB component of the LDR image by a respective RGB component of the original HDR image. Both the LDR image and the FC-frame are separately compressed. In this way a data file transporting the LDR image and the FC-frame can be decoded by a legacy decoder (that will decode only the LDR image), and by a new decoder that, using the FC-frame, will be able to reconstruct the HDR image. U.S. Pat. No. 8,462,194 also discloses scaling and/or gamma to correct the FC-frame; details of these scaling and gamma corrections are not given, e.g. U.S. Pat. No. 8,462,194 discloses that scaling is obtained by multiplying by an “ƒ” factor which is not defined.
Although effective, the method known from U.S. Pat. No. 8,462,194 works well only if the LDR image is obtained by clamping the HDR image. If the LDR image is obtained in a different way, the method may not work properly.